In the conventional solar energy collecting system, water or air is utilized as the working fluid or heat carrier and generally water is utilized because the density and specific heat of air are so small that the electric power required for circulating air in a heat collecting circuit is large in comparison with the case where water is utilized.
Since the solar energy collector is necessarily installed on the outside of a building, it is subjected to atmospheric temperature variations. Further, the temperature of a heat absorbing panel of the collector during winter night becomes lower than the atmospheric temperature by several degrees centigrade due to heat radiation from the heat collecting panel to the air. Therefore, there is a strong probability of the water freezing in the system during winter nights, even in moderate climate areas, resulting in damages to the system itself. In order to avoid this problem on winter nights, there have been proposed many methods which are classified into three groups, namely the addition of an antifreezing agent to the water, the removal of water from the energy collecting circuit and the forced circulation of water through the energy collecting circuit.
In the method involving the addition of an antifreezing agent to the water, the viscosity of the carrier fluid increases, causing an increase in the required pumping power. Further, it becomes relatively difficult to control the pH and density of the carrier fluid, otherwise, it requires much labor and high cost. In addition to these difficulties, when the amount of water to be used in the energy collecting system is large, the amount of the antifreezing agent becomes correspondingly large, causing the cost to be high.
In the method involving removing the water from the circuit, the removal itself is difficult when the circuit takes the form of a closed loop. Even if the removal is possible, the cost of replacement water becomes considerable and the circuit itself tends to corrode due to oxygen dissolved in the replacement water, resulting in a short useful lifetime of the energy collecting system. Further, the cost of an automatic control system therefor is high and a failure of the control system may damage the whole system.
In the method involving forced water circulation, a circulation pump is operated when the temperature of the energy collector is lowered to a predetermined value, so that heat in a heat storage tank is returned to the energy collector. In this method, the amount of heat to be returned to the collector is substantial on a winter night, resulting in that the effective utilization of solar energy becomes impossible and the electric power required to operate the pump increases.
There is another disadvantage inherent to the use of water as the working fluid. Since the heat transfer is performed by utilizing the specific heat of water, there must be a temperature difference between a first point and a second point to which a thermal energy is to be transferred from the first point. On the other hand, it is very difficult to arbitrarily set the temperatures of the solar collector and other load side devices associated therewith respectively. Therefore, even if water, which has the highest specific heat among other substances used, the amount of water becomes large, causing the need for large amounts of power for thermal energy transmission and large size piping.
Further, since lower heat collector efficiency is caused by higher heat collector temperature, the heat collector temperature should not be extremely higher than that needed.
A still further problem encountered in a cooling, heating and/or hot water supplying systems of the solar energy type is heat discharge in the moderate temperature seasons (spring and fall). Since, the size of the collector system installation is determined by the maximum cooling load, the maximum heating load and/or an area on a building permitted to install the system, the thermal energy generally exceeds the desired amount of energy for a building during the moderate seasons unless the building has an exceptionally large hot water supply load which is enough to completely utilize the collected thermal energy. For heating or the hot water supplying, this may occur also in summer. In order to overcome this problem, several methods have been proposed. These methods are to provide a heat storage device, to provide a heat discharge device, to drain down the working fluid from the solar collector, to stop a heat collecting pump so that the fluid is boiled in the collector and evaporated, and combinations of the above, etc.
These methods, however, have disadvantages as below.
In the method in which the heat storage device is used to store excess thermal energy, the storage device is expensive and thus it is impossible to practically install it. In the method in which the heat discharge device is used, the heat collecting pump, a heat discharge pump and a fan therefor, if necessary, must be operated during the heat discharging period. This requires much power and increases the cost of the whole system.
In the method in which water is drained down from the collector, the pipings in the collector are necessarily exposed to air. This is a cause of corrosion of the pipes. Further, when the collector is to be used, the collector must be refilled with water. If fresh water is used for the refill, oxygen dissolved therein causes corrosion of the pipings.
In the method in which the circulation pump is stopped to boil and evaporate water in the collector, impurities in water are condensed, which may become a source of corrosion problems. Further in this method, the portion of water which is evaporated must be refilled, providing the same problems as those in the drain method.